Packaging containers of the single use disposable type for liquid foods are often produced from a packaging laminate based on paperboard or carton. One such commonly occurring packaging container is marketed under the trademark Tetra Brik Aseptic® and is principally employed for aseptic packaging of liquid foods such as milk, fruit juices etc, marketed and sold for long term ambient storage. The packaging material in this known packaging container is typically a laminate comprising a bulk layer of paper or paperboard and outer, liquid-tight layers of thermoplastics. In order to render the packaging container gas-tight, in particular oxygen gas-tight, for example for the purpose of aseptic packaging and packaging of milk or fruit juice, the laminate in these packaging containers normally comprises at least one additional such gas barrier layer, most commonly an aluminium foil.
On the inside of the laminate, i.e. the side intended to face the filled food contents of a container produced from the laminate, there is an innermost layer, applied onto the aluminium foil, which innermost, inside layer may be composed of one or several part layers, comprising heat sealable adhesive polymers and/or polyolefins. Also on the outside of the paper or paperboard bulk layer, there is an outermost heat sealable polymer layer. The heat-sealable polymer layers are preferably based on low density polyethylenes.
The packaging containers are generally produced by means of modern, high-speed packaging machines of the type that continuously form, fill and seal packages from a web or from prefabricated blanks of packaging material, e.g. Tetra Brik Aseptic®-type packaging machines. Packaging containers may thus be produced by the so-called form-fill-seal technology basically including reforming a web of the laminated packaging material into a tube by both of the longitudinal edges of the web being united to each other in an overlap joint by welding together the inner- and outermost heat sealable thermoplastic polymer layers. The tube is filled with the intended liquid food product and is thereafter divided into individual packages by repeated transversal seals of the tube at a predetermined distance from each other below the level of the filled contents in the tube. The packages are separated from the tube by incisions along the transversal seals and are given the desired geometric configuration, normally parallelepipedic, by fold formation along prepared crease lines in the packaging material.
The main advantage of this continuous tube-forming, filling and sealing packaging method concept is that the web may be sterilised continuously just before tube-forming, thus providing for the possibility of an aseptic packaging method, i.e. a method wherein the liquid content to be filled as well as the packaging material itself are reduced from bacteria and the filled packaging container is produced under clean circumstances such that the filled package may be stored for a long time even at ambient temperature, without the risk of growth of micro-organisms in the filled product. Another important advantage of the Tetra Brik®-type packaging method is, as stated above, the possibility of continuous high-speed packaging, which has considerable impact on cost efficiency.
A layer of an aluminium foil in the packaging laminate provides barrier properties quite superior to most polymeric barrier materials. The conventional aluminium-foil based packaging laminate for liquid food aseptic packaging is the most cost-efficient packaging material, at its level of performance, available on the market today. Any other material to compete must be more cost-efficient regarding raw materials, have comparable food preserving properties and have a comparably low complexity in the conversion into a finished packaging laminate.
Hitherto, there are hardly any aseptic paper- or paperboard-based packages for long-term ambient storage of the above described kind available on the market, from a cost-efficient, non-foil packaging laminate, as compared to aluminium-foil laminates, that have a reliable level of barrier properties (e.g. oxygen barrier, water vapour barrier etc) and food preservation properties for long term storage, such as for example more than 3 months.
Among the efforts of developing more cost-efficient packaging materials and minimizing the amount of raw material needed for the manufacturing of packaging materials, there is a general incentive towards developing pre-manufactured films having multiple barrier functionalities, which may replace or complement the aluminium-foil. Previously known such examples are films combining multiple layers, which each contribute with complementing barrier properties to the final film, such as for example films having both a vapour deposited barrier layer and a further polymer-based barrier layer coated onto the same substrate film. Such films, which have been coated at least two times with different coating methods, tend, however, to become very expensive and involve very high demands on the qualities of the substrate film, such as thermal resistance and handling durability.
On the other hand, in order to optimise the packaging laminate, the production of it, and of packaging containers manufactured therefrom, there is an incentive, in addition to lowering the raw material costs, to simplify the structure of the packaging laminate, to decrease the number of conversion steps needed and to provide a packaging laminate that has sufficient barrier and food preserving properties.
Many so-called barrier films are provided commercially today. A common denominator for most such films is that they often are too expensive, since they require comparatively thick layers, alternatively or additionally several layers, of precious barrier materials and/or are not good enough in terms of the barrier properties and mechanical properties required for incorporation into a carton packaging laminate, from which fold formed, sterilised, filled and sealed packages (form-fill-seal) are to be produced. For example, a single layer barrier layer of a barrier polymer such as ethylene vinyl alcohol (EVOH) or polyamide for the purpose of high barrier properties, is far too expensive.
One type of such barrier films are so-called high surface energy films (HSE) for subsequent further barrier coating with ceramic, organic or metallic vapour deposition coatings, such as SiOx coatings or metallisation coatings. The high surface energy of the film, mostly based on polypropylene or similar polyolefin films, is provided by a thin surface layer of e.g. polyamide or ethylene vinyl alcohol.
In EP-B-541273, first filed in 1991, there is described a barrier film wherein an aqueous coating dispersion comprising polyvinylalcohol and an adhesion promoting, co-polymer or modified polymer, is coated onto a polypropylene substrate film, which has been oriented in a first direction. After the coating and drying operation of the PVOH-based coating, the film is subsequently oriented in the second direction, in order to produce a biaxially oriented film, having a surface suitable for subsequent further metallisation onto the PVOH-based surface. The costs of such a film are, however, very high, since they involve two coating steps of different kinds, first a wet dispersion coating with subsequent drying, and there after a further vapour deposition coating operation, with an orientation operation between these two coating operations. There is hardly any, or no, economic advantage of including such a film into a laminated material for disposable packaging containers.
In U.S. Pat. No. 5,153,074 (first filed in 1991), a film for metallisation having a high energy surface of EVOH is described. The base layer of polypropylene is coextruded together with the EVOH surface layer with a bonding layer of a maleic acid anhydride modified polypropylene homopolymer in between the two layers. According to the only Example, the thus obtained film is then sequentially oriented, first three times its original length in the machine direction and then 8 times in the transversal direction. The EVOH employed in the Example had an ethylene mole percentage of 48. The final film total thickness was 80-100 gauge units, while the thickness of the EVOH layer was only 3 gauge units. According to the measurements on the metallised film of the Example, the oxygen transmission was from 2.6 to 5.4 cm3/m2/day/atm at 0% RH and 23° C.
Obviously, the very thin layer of EVOH in this film serves only as a metal-receiving layer and not actually as an oxygen barrier layer. Especially, since the EVOH employed has a high content of ethylene monomer units, thus having rather low inherent gas barrier properties.
In tests by applicants of the present invention, high-surface energy uncoated substrate films, such as described in U.S. Pat. No. 5,153,074, provide oxygen transmission rates as high as from 70 to 110 cm3/m2/day/atm at 23° C. 50% RH.
In US-A-2009/0053513 (corresponding to WO2006/117034, first filed in 2005), a similar (to U.S. Pat. No. 5,153,074) BOPP-based film having a high surface energy layer of polyamide, for subsequent coating with SiOx, AlOx or a metallised coating, is described to provide surprisingly improved oxygen barrier properties in its barrier coated state, compared to the previous structures. The improvement is explained to be related to the method of simultaneous biaxial orientation (LISIM®), wherein the stretching of the polymer film is carried out simultaneously in the MD and TD to at least a stretching ratio of above 6-7 times the original length and width of the film material. The oxygen transmission values obtained by such a further barrier coated film is claimed to be lower than 0.20 to 0.50 cm3/m2/day/atm at 23° C. 75% RH. Also in this case, however, the oxygen barrier obtained is related to the subsequent barrier coating, rather than to the HSE-type substrate film itself.
It is generally known that films from EVOH copolymers are difficult to orient and stretch. It is believed that this is due to the large number of hydroxyl groups in the molecules of EVOH, which easily form hydrogen bonds during formation of the non-oriented film.
In US-A-2009/0208717 (corresponding to WO2006/128589, first filed in 2005), the method of simultaneous biaxial orientation (LISIM®) is used to stretch films with symmetrical configuration and internal EVOH gas barrier layers. Here, a clear improvement of the EVOH layer barrier properties is seen. The EVOH barrier properties were increased two-fold (doubled) compared to similar non-oriented films having the same layers and layer thicknesses. Also by this publication, it is taught that by sequential biaxial orientation of similar films, the ethylene content of the EVOH polymer must be higher than 45 mole-%, and that also simultaneous biaxial orientation of films with internal EVOH layers having a lower ethylene content than 40 mole-% was earlier considered impossible. According to the invention as described in US-A-2009/0208717, however, simultaneous biaxial orientation of a film having a central EVOH layer was found possible, also at ethylene contents below 40 mole-%, conditional certain temperature and stretching conditions. US-A-2009/0208717 discloses that it is possible to simultaneously stretch a film of the general structure B/C/D/C/B, where the two B layers are based on polypropylene homopolymers, the two C layers are polypropylene or polyethylene modified by maleic anhydride, and layer D is the above specified central EVOH layer. The thickness of the EVOH layer should generally be from 1 to 10 μm, preferably from 1 to 6 μm. Furthermore, it is well known, and also taught by US-A-2009/0208717, that EVOH layers should be protected from the environment as their barrier properties are impaired on ingress of atmospheric moisture. Thus, the EVOH layers are arranged in the core of a multi-layer film (B/C/D/C/B). Moreover, it is taught by US-A-2009/0208717 that also film blowing methods such as so-called “Bubble” or Double-bubble” methods are included in the range of simultaneous orientation methods.